Photodetectors often function either as photodiodes or as photoconductors: photodiodes offer fast response and low dark current, while photoconductors provide built-in gain associated with the use of long-lived sensitizing centers. The concept of a phototransistor, such as a photo-field-effect-transistor, offers an attractive possibility: gain united with a lowered dark current compared to the photoconductor, achieved if the thickness of the current-carrying channel can be chosen independently from the thickness of the light-absorbing layer.